1. Field of the Invention
This invention relates to a gauge for determining the thickness of nonmetallic articles, more particularly a gauge which employs the principles of an air core transformer whose coefficient of coupling depends upon the distance between two electrical coils.
2. Description of the Prior Art
There are numerous applications where precise measurement of article thickness is desirable. The measurements are difficult where the article has an irregular shape and a thickness more than about 1 inch. One example is in measuring the thickness of glass fiber reinforced plastic dome structures. Such structures are hemispherical and may have a diameter of perhaps 30 feet. The structures are fabricated from glass fiber reinforced plastics and plastic-impregnated honeycomb reinforcement and have a thickness of about 2 inches. Uniformity of thickness is critical in such structures to assure that the domes present a uniform "window" for radiant energy waves passing therethrough. Conventional measuring means are not satisfactory for measuring the thickness of such hemispherical structures.
For articles having a thickness of 1 inch or less, there are available measuring instruments employing Eddy current principles which are accurate and precise for thickness measurements up to about 1 inch. The use of microwave techniques for thickness measurements of nonmetallic articles has exhibited variations in precision resulting from variations in the dielectric properties of the article which is measured.
A known measuring device employed a pair of electrical coils, positioned one on each side of an article to be measured. An alternating current signal, approximately 10 kilohertz, was applied to one coil and the signal from the other coil was measured. By prior calibration of the two coils, functioning as an air core transformer, it was possible to determine the distance between the two coils--and hence the thickness of the test article--by referring to the previously obtained and recorded calibration data. The air core transformer principle was independent of small variations in the dielectric composition of the article to be measured but its accuracy was affected by changes in temperature and by small amplitude or frequency variations in the signal source. By employing a constant temperature environment for measurements and by employing stable signal generators for the system, accuracies of .+-.0.010 inches at a 2 inch measurement distance were achieved.
Improved accuracy in measurement is desirable. A measuring gauge which does not require constant temperature environment or extremely stable signal generators also is desirable.